It is well known that many items are preferentially packaged in plastic films, particularly transparent plastic films. However, when the items to be packaged are of variable size and shape, such as, for example, vegetables and fruits, the presently known mechanical packaging operations may present considerable difficulties.
Plastic films have been generally known before the Second World War, but immediately after the war, the use of plastic films for packaging increased rapidly. A variety of polymers were used in order to provide packaging films having improved properties.
Prior to 1958, the films most generally in use for packaging were polyvinylchloride, polyethylene, polypropylene, and polyvinylidene (Saran), alone or mixed with different elastomers. Plastic films made from these polymers cannot be considered to be stretchable films because these films have a low extensibility. Films made from these polymers have a longitudinal or transversal extensibility which is less than 50% at normal temperatures (from about 5 to about 35.degree. C.).
Thus stretchable films, under the same temperature conditions as above, have an extensibility of at least 50%, and in some cases up to about 300%. Another striking difference between non-stretchable and stretchable plastic films is the elastic memory of the films. Normal films, even when stretched for about 35 to 45%, do not return to the original dimensions, but remain over-extended and weakened. Stretchable films, on the other hand, return almost to the original dimensions after being stretched, and do not lose the mechanical properties they had before being stretched.
The machines presently available for packaging articles in plastic films may be categorized in two groups: those using an unstretchable tubular plastic film, and those using a stretchable flat plastic film. In the first case, the machine inserts the item to be packaged into the tubular film, then the film bottoms sealed to form a bag, which is then sealed by welding or fastening. In such packages, however, the film adheres only onto very few parts of the surface of the item, and, consequently, there are formed recesses and irregularities which may cause the package to split apart during subsequent handling of the packaged item. The mechanisms wherein the flat plastic film is of the stretchable type first stretch the film and then wrap the item around the item, lapping the film edges in the area where the film is to be sealed. In this case the packages are also irregular in shape, and excess plastic material is often left on the package. Because of these and further difficulties, in many installations packaging using a stretchable film is performed manually, in order to benefit from the advantages shown thereby.